Creating A True Singleton In Node.js, With ES6 Symbols

Creating A True Singleton In Node.js, With ES6 Symbols

// https://derickbailey.com/2016/03/09/creating-a-true-singleton-in-node-js-with-es6-symbols/

// create a unique, global symbol name
// -----------------------------------

const FOO_KEY = Symbol.for("My.App.Namespace.foo");

// check if the global object has this symbol
// add it if it does not have the symbol, yet
// ------------------------------------------

var globalSymbols = Object.getOwnPropertySymbols(global);
var hasFoo = (globalSymbols.indexOf(FOO_KEY) > -1);

if (!hasFoo){
  global[FOO_KEY] = {
    foo: "bar"
  };
}

// define the singleton API
// ------------------------

var singleton = {};

Object.defineProperty(singleton, "instance", {
  get: function(){
    return global[FOO_KEY];
  }
});

// ensure the API is never changed
// -------------------------------

Object.freeze(singleton);

// export the singleton API only
// -----------------------------

module.exports = singleton;

SingletonDefaultExportInstance.js

class SingletonDefaultExportInstance {
  constructor() {
    this._type = 'SingletonDefaultExportInstance';
  }

  singletonMethod() {
    return 'singletonMethod';
  }

  static staticMethod() {
    return 'staticMethod';
  }

  get type() {
    return this._type;
  }

  set type(value) {
    this._type = value;
  }
}

export default new SingletonDefaultExportInstance();

// ...

// index.js
import SingletonDefaultExportInstance from './SingletonDefaultExportInstance';

// Instantiate
// console.log(new SingletonDefaultExportInstance);  // is not a constructor

// Prototype Method
console.log(SingletonDefaultExportInstance.type, SingletonDefaultExportInstance.singletonMethod());

// Getter/Setter
SingletonDefaultExportInstance.type = 'type updated';
console.log(SingletonDefaultExportInstance.type);

// Static method
console.log(SingletonDefaultExportInstance.constructor.staticMethod());